Objectives The objective of this study was to analyze the impact of the abutment width on early marginal bone loss (MBL). Material and Methods A balanced, randomized, double-blind clinical trial with two parallel experimental arms was conducted without a control group. The arms were “cylindrical” abutment and “concave” abutment. Eighty hexagonal internal connection implants, each with a diameter of 4 × 10 mm, were placed in healed mature bone. The main variable was the peri-implant tissue stability, which was measured as MBL at 8 weeks and 6 months. Results The final sample consisted of 77 implants that were placed in 25 patients. 38 (49.4%) were placed using the cylindrical abutment, and the other 39 (50.6%) were placed using the concave abutment. The early global MBL of −0.6 ± 0.7 mm in the cylindrical abutment group was significantly higher than it was in the concave abutment group, in which the early global MBL was −0.4 ± 0.6 mm (p = .030). The estimated effect size (ES) was negative for the cylindrical abutment (ES = −1.3730, CI −2.5919 to −0.1327; t-value = −2.4893; p = .0139), therefore implying a loss of mean bone level, and it was positive for the concave abutment (ES = 2.8231; CI: 1.4379 to 4.2083; t-value = 4.0957; p = .0002), therefore implying an increase in the average bone level. Conclusions The concave abutments presented significantly less early MBL at 6 months post-loading than classical cylindrical abutments did.